Increased genomic instability is a hallmark of aging. Trinucleotide repeat (TNR) expansions are one type of instability; these unstable expansions contribute to over a dozen neurodegenerative diseases. Unfortunately, these diseases are difficult to study in vivo because they are characterized by late onset, long time course and a complex phenotype. While Friedreich's Ataxia (FRDA) has the same underlying mechanism as age-related TNR diseases, it has an accelerated course and can be studied in vitro. The contribution of the DNA mismatch repair (MMR) pathway to age-related TNR expansion is an area of intense focus. Using FRDA as a model, we can determine what role DNA mismatch repair proteins play in age-related TNR diseases. In the MMR pathway DNA mismatches are recognized and repaired in a stepwise fashion by MutS and MutL heterodimers. Our preliminary data suggests that silencing the MutS Homologue 3 (MSH3), a component of the MutS complex, slows the rate of expansion. Therefore, we hypothesize that depleting MSH3 will slow the rate of TNR expansion and that the MutL pathway is a necessary component of that expansion. Specific Aim 1: we will determine the role of MSH3 in repeat expansion through overexpression and the use of both lentivirus- mediated and zinc finger nuclease-mediated knockdown of MSH3. Specific Aim 2: we will use the same methods to establish the contribution of the MutL complex proteins (MutL Homologue 1 (MLH1) and MLH3) to the rate of repeat expansion. This research is significant because it provides insight into the mechanism of a common type of genomic instability that underlies age-related neurodegeneration. Such insight could provide new targets for the development of more effective treatments.